A special bumper was installed on selected vehicles in a large fleet. The dollar cost of body repairs was recorded for all vehicles that were involved in accidents over a 1-year period. Those with the special bumper are the test group and the other vehicles are the control group, shown below. Each "repair incident" is defined as an invoice (which might include more than one separate type of damage.)

Statistic Test Group Control group

Mean damage x1=$1,101 x2=$1,766

Sample standrd dev s1= 696 s2= $838

Repair Incidents n1= 12 n2 =9

a.) Construct a 90 percent confidence interval for the true difference of the means assuming equal variances. (Round your answers to 3 decimal places. Negative values should be indicated by a minus sign.)

The 90% confidence interval is from ? to ?

B.) Repeat part (a), using the assumptions of unequal variances with Welch's formula for d.f.(round your answers to 2 decimal places. Negative values should be indicated by a minus sign.

The 90% confidence interval from ? to ?

d.) Construct seperate 90% confidence intervals for each mean. (round your answer to 2 decimal places.)

mean damage confidence interval

x bar1 = $1.101 ? ?

xbar2 = $1,766 ? ?